Allergic diseases affect an increasing number of people in the United States and worldwide. Among these diseases, allergic asthma is particularly concerning because of its high morbidity and financial burden. While medical science has made profound advances in the treatment of asthma symptoms, a true cure for the disease remains elusive. Many researchers seeking a cure for asthma have focused their attention on the immune system. In asthma, the immune system reacts inappropriately to common, harmless products (allergens) in the environment, such as house dust mite (HDM). Normal individuals do not experience an immune response when encountering such allergens and are said to be tolerant to the allergen. However, asthma suffers lack this tolerance and experience a damaging inflammatory response. Understanding how tolerance develops and why it is lacking in asthma is critical for development of a true cure. The immune system possesses regulatory elements that help control immune responses. Previous work has shown that one of these, the regulatory T cell (Treg), plays a key role in the establishment of tolerance to allergens. However, the mechanisms by which Tregs are generated and prevent asthma in healthy people remains unknown. One possible influence on the immune system is the microbiome, the community of microorganisms that live on and within each of us. Over the past decade, researchers have discovered that the microbiome can exert regulatory effects on the immune system, including induction of Tregs. Many studies have shown that loss or disruption of the microbiome promotes allergy and asthma; thus, the microbiome may have a key influence on tolerance development. This proposal hypothesizes that the microbiome promotes tolerance to allergens like house dust mite (HDM) through promotion of Treg formation. The overall goal of this proposal is to determine the effect of HDM exposure on the microbiome and the relationship of the microbiome to allergic disease and tolerance. Overall Impact: Completion of the proposed aims may reveal new opportunities for microbiome-based therapies that could provide curative treatments for asthma.